Mark scanning or optical mark reading devices are known which are based on the principle of optical detection of a mark on the surface of a document due to the fact that the light reflected from a mark is less than the light which would be reflected by the same surface without marks. The common approach is to set up a light source, such as an electric light bulb, for example, near the surface of the document, together with a light sensor, such as a photodiode.
The photodiode measures the light reflected by the mark and converts this quantity to an electrical current; this current must exceed a certain threshold to be taken into account and indicate the presence of a mark.
There are a number of disadvantages associated with the prior art devices however. The light source, placed near the document, radiates heat which could damage the document. With a moving document, on a conveyor for example, the document to source and/or sensor distance has been observed to vary, bringing about a variable electrical response from the sensor. This results in a fuzzy determination of the threshold, known as the mark detection threshold, at which the electrical signal delivered by the photodiode should be taken into account. Also, differences in the document's background color or shade as well as differences in print quality of the markings result in variable sensor response. Such variable mark reading conditions can lead to photodiode electrical response minima for which the marque detection threshold is not always attained. The operator is then required to manually adjust the threshold according to the reading conditions, by guessworking a potentiometer. In actual operation, the light source may also exhibit small changes in light intensity due to temperature or power supply instabilities.
All of these factors are a source of numerous errors in the interpretation of data read from the documents.
Some optical mark readers have been developed in the prior art to minimize the above-mentioned drawbacks, especially the need to adjust the mark detection threshold, making use of two photosensors coupled to a differential output amplifier and comprising means for further processing of the resulting differential signal. The optical mark reader described in French patent application No. 81 08886 by the same assignee is based on such a differential measurement principle for mark recognition.
Moreover, it removes the light source and preferably also the photosensors some distance away from the documents to reduce their incidences and thus equally minimize possible errors and preclude possible damage to the documents. To this end it uses three sets of optical fibers, with first ends of one of the sets of fibers coupled by means of optical connectors to the light source, the first ends of the other two sets of fibers coupled respectively to each of the two photosensors, the opposite ends of the three sets defining a light-emitting head and two flanking receiving heads facing the document. The receiving heads are mutually arranged so that they simultaneously receive light reflected by two parallel areas of the document, said areas being as close to one another as possible so as to be equivalent in terms of the document's background shading, yet distinct enough that one and only one of them carries the series of marks to be read whilst the other is blank. Said heads are assembled on a support purposely provided with three parallel positioning slits for their accommodation.
In the optical reader according to the above-mentioned patent application, the two photosensors or photodiodes, as well as separate amplifiers for the two photodiode signals and the optical connectors used to couple the optical fiber sets to the photodiodes are mounted on an analog input card. The electrical signals from the photodiodes are transmitted after amplification over electric cables to a digital processing system.
The present invention is directed to an optical reader head based, as in the previously mentioned patent application, on the principle of a differential input for mark reading, with the removal some distance away from the documents of both the light source and the photosensors. Its main object is to specify a rugged, yet compact, low-cost optical reader head obviating the problems associated with mass production of the previous head unit and with its fragility.
The invention thus provides an optical reader head for document markings comprising a light source and two photosensors, all three removed from the scanned document and coupled to light transmission means defining a set of three individual terminal heads arranged side by side and parallel to one another in front of said document, one of said terminal heads being light-emitting and being bracketed by said two other, light-receiving heads, and further comprising an analog circuit for processing the electrical signals transmitted by said photosensors, said reader head being characterized in that said source, said photosensors, said light transmission means and said analog processing circuit board are grouped together and positioned relative to one another in a same flat supporting case, said case being internally divided by a ridge going substantially through the center thereof, parallel to the two small opposite faces or front and back faces thereof, into a front section and a back section, said front section being provided with three rigid, preformed optical conduits forming said light transmission means spaced apart by sets of ribs in said case defining their respective channels to form three parallel slits in said front face, substantially crossing the center portion thereof, said two light-receiving side channels fanning away from one another as of substantially the front face, said back section carrying said analog signal processing circuit, on which are mounted, on the same side thereof, said two photosensors, arranged in cavities provided in said central dividing ridge, into which cavities issue said respective side channels, and said light source, said latter source being connected to said circuit at least through a cutout in said central ridge.
According to one embodiment of the invention, said rigid optical conduit or lightwave guide located in the center channel termed the emitting conduit is given a coupling end of circular cross section for coupling with said source and its other end, forming said light-emitting head, is given a rectangular cross section, the longer dimension thereof being substantially the same length as the cross-sectional diameter of said coupling end.
According to another embodiment of the invention, said rigid optical side or "outside" channels, termed the light-receiving conduits, are given a coupling end of either square or rectangular cross section for coupling with one of said photosensors and their other end, forming said receiving head, is given a rectangular cross section one of the sides whereof is smaller than its corresponding dimension on the photosensor coupling end.
Advantageously, the cross sections of the coupling end and of the end forming the receiving head of said light-receiving outside conduits are substantially equal. They are furthermore preformed in the shape of a slanted S. Said optical conduits are made of molded plastic, for example of polymethacrylate or polycarbonate (PC).
In still another embodiment of the invention, said light source is located in said front section of the supporting case and a window is provided in at least one of the broad sides of said case in front of said source.
The light source is preferably mounted in a heat sink extending throughout the height of said window transversely to said front face of the case and held in said cutout in the center ridge and in a recess provided in line therewith in the ribs bounding said center channel, said heat sink acting as an extension of said center channel.
In yet another embodiment of the invention, said analog processor board is locked in place within said back section of the case by a stop provided on the back wall thereof and by said photosensors pressing against said coupling ends of said light receiving conduits.
Said supporting case is preferably made of two identical mating halves.